Aluminum oxide coated cemented carbide cutting tools have been in industrial practice for well over 15 years and are today commonly used for the turning and milling of cast irons and steels.
Al.sub.2 O.sub.3 coatings are usually deposited by chemical vapor deposition (CVD) after applying an intermediate layer of TiC, TiN or Ti(C,N), either as a single layer or as multilayers onto the cemented carbide substrate. Instead of using intermediate layers, the cemented carbide substrate can be enriched with .gamma.-carbide phase, i.e., a solid solution of cubic carbides of titanium, tantalum, niobium and tungsten, prior to Al.sub.2 O.sub.3 deposition. In order to enhance adhesion of alumina to the intermediate layer, one or several interlayers, usually referred to as bonding layers, can be applied between the intermediate layer and the alumina layer. Furthermore, the alumina coating itself may be deposited as multilayers. Accordingly, a plurality of coating combinations exist.
The CVD alumina layers are usually composed of pure .kappa.-Al.sub.2 O.sub.3, mixtures of .kappa.-Al.sub.2 O.sub.3 and .alpha.-Al.sub.2 O.sub.3 or pure .alpha.-Al.sub.2 O.sub.3. A layer of TiN is usually deposited on top to the alumina layer. The TiN layer is usually relatively thin (1 to 2 .mu.m) and is mainly used to give the tool an attractive appearance--the so-called "golden color". For example, the TiN layer, which is referred to as a finish layer in U.S. Pat. No. 4,984,940, is used to provide a low friction surface and to minimize metal build-up on the coating. Accordingly, the TiN in this case is relatively thin, about or less than 2 .mu.m and much less than 4 .mu.m, although it is disclosed in this patent to be 0.2-4 .mu.m.
In addition to the golden colored alumina coated inserts, black inserts, i.e., alumina coating without the TiN top coat, are present in the marketplace and have, in fact, gained great industrial success. These inserts are often wet blasted in order to enhance the chipping resistance and surface finish. It has earlier been assumed that wet blasting must be performed directly on the alumina coating for two main reasons:
1) The TiN layer deposited on top of the alumina layer is assumed to deteriorate the wet blasting effect; and
2) Wet blasting of TiN coated alumina inserts cannot be controlled in production scale due to the poor adhesion between TiN and alumina. For this reason, there have not been any wet blasted TiN coated or Al.sub.2 O.sub.3 -TiN coated inserts on the market.
Consequently, the TiN coated alumina inserts are usually brushed in order to enhance the chipping resistance and edge strength. Brushing of the TiN coated (golden colored) alumina inserts results in a black line along the cutting edge (area where the TiN layer is worn away).